Hook and loop fastenings are based on the way that a cocklebur sticks to a wool sock. (see Velcro for the story)

There are two pieces, a hook piece and a loop piece. The hook piece has rows of tiny plastic hooks, based on those on a cocklebur. The loop side looks like tiny loop-pile carpet, made of fine thread.

When you press the two pieces together, each hook catches on a bit of thread in the loop side. The plastic of the hooks is thin enough that they can straighten up. As they straighten, they release the loops with a slight tearing sound.